Add design system spec for the upcoming UI rework

Moved into .claude/context/ (alongside the other topic docs CLAUDE.md
points agents at) rather than the repo root, so it's discovered the same
way as architecture.md/styling.md/etc. Flags styling.md as documenting the
pre-rework theme in the meantime, so the two don't read as contradictory.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CLAUDE.md

@@ -15,6 +15,11 @@ relevant file(s) before working in that area rather than loading all of them:
   conventions, fonts, reusable chrome widgets
 - [`standards.md`](.claude/context/standards.md) — code style, comment
   conventions, linting, testing
+- [`design-system/DESIGN_SYSTEM.md`](.claude/context/design-system/DESIGN_SYSTEM.md)
+  — the target visual/component spec (tokens, components, platform
+  mapping, screen recipes) for the in-progress UI rework; the sibling
+  `.dc.html`/`noo-kit.js` files it references are the visual/token
+  reference builds, not something to read directly
 
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